Chapter 5
Ladder Program Basics
5–9
Application Example
Use the following program to achieve the maintained contact action of an
On–Off toggle switch using a momentary contact push button. (Press for
On; press again for Off.)
The first time you press the push button (represented by address I:0/1),
instruction B3/11 is latched, energizing output O:0/2. The second time you
press the push button, instruction B3/12 unlatches instruction B3/11,
de–energizing output O:0/2. Instruction B3/10 prevents interaction between
instructions B3/12 and B3/11.
Status Bit
I:0/1
Rung
B3/10
B3/12
O:0/2
B3/11
1
2
3
4
]/[
B3
10
1
] [
I:0.0
1
( )
B3
11
( )
B3
12
2
] [
B3
11
]/[
B3
12
]/[
B3
10
( )
2
( )
B3
10
] [
I:0.0
1
] [
B3
11
3
4
] [
B3
11
] [
I:0.0
1
] [
] [
] [
( )
]/[
]/[
( )
] [
] [
] [
( )
( )
]/[
O:0.0
As previously indicated, the processor executes instructions individually,
rung by rung, from the beginning to the end of the program. This is called a
program scan and it is repeated many times a second. The figure on the next
page indicates in greater detail what happens during individual scans when
an external input device (represented by I:0/1) is operated.
